1. Field of the Invention
This invention relates to the field of OA (office automation) devices, such like a copying machine, a facsimile, a laser printer and so on, and in particular to an apparatus for acoustic noise reduction of the OA devices.
2. Description of the Prior Art
According to the development of technology and the demand for an improvement of the efficiency of office work, OA devices, such as a copying machine, a facsimile, or a laser printer and so on, are becoming more popular in many office environments.
In the operation of OA devices, acoustic noise is generated. In some situations, the noise might reduce an efficiency of the office work. Due to increased use of OA devices, it becomes more important to reduce the acoustic noise generated by OA devices for efficient office work.
Generally, one effective solution for reducing acoustic noise is to prevent the leakage of the noise from a noise source by sealing the noise source.
A conventional device for reducing acoustic noise generated from the OA devices is disclosed in Japanese Patent Laid-Open Publication No. 4(1992)-469. This publication discloses a laser printer and is designed to reduce the noise which is generated by a gear member which adjoin the image development unit. According to this publication, this laser printer is provided with a soundproof shield which covers the gear member which adjoins the development unit, and a hollow type of silencer member is provided in the hole of the soundproof shield to seal up the noise source, such as the gear member.
But the OA devices usually have an inlet hole for cooling air, sometimes with a fan unit. Furthermore, they require openings through the outer shield of the OA devices to feed a paper from a paper storage unit to a image development unit therein, and to output the paper therefrom after image development has been completed. These openings leak acoustic noise generated by the noise source in the OA device, such as a gear member or a cooling unit. The abovementioned device does not account for the openings necessary for a flow of air for cooling the inside of the device and the paper feed operation of the device. For these reasons, it is hard to effectively reduce the acoustic noise of the OA devices using the above mentioned devices.
Another apparatus is disclosed in Japanese Patent Laid-Open Publication No. 4(1992)-221965. It discloses an image forming apparatus comprising a system having an active noise cancellation system. But this system requires expensive components to control the noise of the image forming apparatus. The system needs a microphone unit to input the acoustic noise information, a speaker unit to output corrective acoustic signals, and high performance signal processing unit with memory. So this apparatus does not have adaptability to general OA devices and could not be used for low cost OA devices.
Another known device is shown in FIG. 13a and 13b. This apparatus utilizes an acoustic noise insulation device, such like a glass wool or a forming member. This apparatus is provided on an outer shield 71 of the OA device, which shield is provided a hole 71a for the flow of the air. It includes a cover member 72 which covers the hole 71a. The cover member 72 has an internal volume with an open end. Although the device of FIGS. 13a and 13b is not described as an acoustic noise insulation device, the cover member 72 is covered with internal insulation and so acts as a silencer. It permits air flow within the OA device and so reduces the transmitted noise to outside of the OA device.
As well known in this technical field, the frequency band for which the acoustic noise insulation device is effective is near 1 kHz. But the frequency of most powerful acoustic noise generated by the cooling fan unit is in the neighborhood of about several hundred Hz. This figure is a multiple of the number of fan blades and the rotational speed of the fan. A main noise frequency generated by the gear member which adjoins the image development unit is also several hundreds Hz.
So this type of silencer is ineffective for attenuating acoustic noise in the frequency band of the main acoustic noise generated in the OA device, and it is not adequate for noise reduction.